The age of megacities

More than half of Earth's population lives in cities—and many of them reside in megacities, the world's largest urban aggregations.

The world is a crowded place, with human population expected to reach 9 billion by mid-century. Over half of us now live in urban areas, the largest of which have swelled into giant urban agglomerations, sometimes called megacities—metropolitan areas with 10 million people or more (mapped on the globe below).

In 2020 the world will have 34 megacities,  according to the United Nations , and that figure is projected to increase. Scroll down to explore the historic growth of 10 of today’s largest cities.

Tokyo, Japan

2020 Population: 37 million | Rank: 1

Although Tokyo today is the world’s largest city, young residents are having fewer children, leading to population declines that may see the city proper reduced to  7.13 million by 2100 . Nearly 3.27 million of those estimated residents will be over the age of 65, making it difficult for Tokyo to have a large enough workforce to pay for health care and pensions for the elderly.

The final map in these series shows 2016 population density based on analysis of remote sensing imagery. See sources at the end of this story map.

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Shanghai, China

2020 Population: 27 million | Rank: 3

As a crucial port city, Shanghai already had more than 500,000 residents by 1816, and it is now the most populous city in China. Infrastructure has largely been able to keep up with demand, as most residents have access to water, electricity and waste collection. But the population density does present environmental problems. For instance, air pollution from the city’s dependence on coal  plagues public health .

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São Paulo, Brazil

2020 Population: 22 million | Rank: 4

It’s not just size but population distribution that makes São Paulo distinctive. Unlike some megacities, which concentrate population in an urban center, many of the city’s residents have moved into the  farthest suburbs . Many workers living here commute into the city center, adding to pollution. And as São Paulo’s fringes continue to expand, they erode surrounding forests and other natural landscapes.

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Mexico City, Mexico

2020 Population: 22 million | Rank: 5

Mexico City’s population skyrocketed between 1940 and 1995. Much of this growth was tied to growth of industries in the city center, which offered better access to electricity, water and gas. But expansion came with serious infrastructure problems. Though the city has adequate water capacity, it lacks an efficient distribution system, and those living on the outskirts are often forced to use untreated wastewater.

Cairo, Egypt

2020 Population: 21 million | Rank: 7

Cairo tops the list of largest cities in Egypt, Africa, and the Arab world. It sprawls along both sides of the Nile River at the base of its delta; the city's dense urban landscape echoes with blaring car horns and amplified calls to prayer from its scores of mosques.

A large proportion of the city's population lives in poverty. Estimates of the number of people in Cairo's slums ranged as high as  10.7 million in 2006 . Cairo continues to grow rapidly, although its  rate of growth  has slowed from nearly 4 percent in 1996 to just over 2 percent today.

Beijing, China

2020 Population: 13 million | Rank: 8

Beijing is China's capital, its second largest city (after Shanghai) and the nation's most important political, cultural, and business hub. Beijing's population is growing at a rate of  about 2 percent annually ; meanwhile its per capita GDP has been increasing by 8 to 10 percent, reflecting China's continuing economic development.

Lagos, Nigeria

2020 Population: 14 million | Rank: 17

While Lagos is the largest city in sub-Saharan Africa,  two out of three  of its residents live in slums, lacking viable access to clean water, waste disposal, electricity and roads. And Lagos’s population only continues to grow due to high birthrates. By 2030 Lagos will jump to number 11 on the list of world megacities.

Los Angeles, USA

2020 Population: 12 million | Rank: 25

The City of Angels started out as a town of just 4,000 people. By the late 1800s the transcontinental railroad was delivering droves of new residents, driven in part by a  marketing campaign  that billed L.A. as a luxurious paradise. In the Roaring Twenties, the booming middle class became enamored with automobiles—between 1920 and 1930, the number of gas stations increased from 170 to more than 1,500.

Paris, France

2020 Population: 11 million | Rank: 28

Paris has been a  permanent human settlement  for more than two millennia. In the 1850s, post-Revolution leaders transformed the city, widening roads, planting trees and improving the sewage system. So Parisian farmers came up with a way to feed the booming population: using manure from the many horse-drawn carriages as compost. New urban farms yielded 110 pounds of produce per resident annually. Eventually cars replaced horse carriages, and urban farming in Paris declined.

London, England

2020 Population: 10 million | Rank: 35

In 1815 London was the  largest city in the world . Today it no longer ranks among the largest even in Europe (that honor goes to Moscow). But an influx of newcomers and young families is set to push the population over 11 million by 2050, and that rapid growth will strain London’s existing  infrastructure . Left unchecked, demand for water will outpace supply by 21 percent.

The world's megacities in 2030


Largest megacity: Delhi, India | Projected population: 39 million

As the population of Tokyo declines due to an aging population, Delhi's rapid growth will result in its becoming the world's most populous city by 2030.

The graph shows the projected 10 largest megacities in 2030 (brighter carets) vs. 2020 (darker carets). Cities in Africa, the Middle East, and South Asia are undergoing rapid growth (red bars), while growth rates are slowing in other parts of the world.

The map shows world cities with projected 2030 populations of more than 10 million. Click on circles for details.

Will the growth of megacities worsen climate change, loss of biodiversity, and pollution? Or will the concentration of large populations in small areas provide opportunities for increased sustainability and environmental health? The answer: It depends.

Cities are traditionally centers for innovation. The World Economic Forum has outlined  10 Urban Innovations  that could make cities more sustainable and economically prosperous. Scientific American's " Science for Sustainable Cities " report examines how smart urban planning and design can help people and the planet.


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